1. Field of the Invention
The present invention relates to the manufacture of a semiconductor device, more particularly to a method of forming a bottle-shaped trench in a semiconductor substrate for formation of a capacitor for dynamic random access memory.
2. Description of the Related Art
As the integration density of a dynamic random access memory (DRAM) steadily increases, it becomes necessary to reduce the memory cell size. The memory cell size is primarily determined by the minimum resolution dimension of a lithographic technique, the overlay tolerances between the different features and the layout of these features. At the same time, it is necessary to maintain the minimum required storage capacitance to reliably operate the DRAM. To meet both the cell size requirement and the storage capacitance requirement, a trench capacitor was invented; the simple single device/capacitor memory cell has been altered to provide the capacitor in a vertical dimension. In such designs, the capacitor is formed in a trench in the surface of the semiconductor substrate. For example, U.S. Pat. No. 5,348,905 issued to Kenney on Sep. 20, 1994, entitled, “METHOD OF MAKING DIFFUSED BURIED PLATE TRENCH DRAM CELL ARRAY”, teaches the basic elements and process steps for fabricating a buried plate DRAM cell structure.
However, as the size of a DRAM is scaled down by a factor of f (feature size), the trench storage node capacitance decreases by a factor of f. Therefore, it is important to develop methods to increase the storage capacitance. One method employed to increase capacitance is to widen the bottom portion of the trench, thus, increasing the surface area and creating a “bottle shaped” capacitor. However, in order to space the capacitors close together, control of the etching process used to widen the bottom portion becomes a governing factor. Chemical dry etching is predominantly used in the prior art to create the bottle-shaped portion of the capacitor. In U.S. Pat. No. 5,112,771 issued to Ishii, et al. on May 12, 1992, entitled, “METHOD OF FABRICATING A SEMICONDUCTOR DEVICE HAVING A TRENCH”, the bottom region of a trench capacitor is enlarged. This is accomplished by leaving a silicon oxide film on the upper sidewall of a trench, and enlarging the width of the exposed bottom portion of the trench by an isotropic dry etching process. Since the silicon substrate is isotropically dry etched, it is etched not only in the vertical direction to the surface of the substrate, but also in the horizontal direction. Although the capacitor surface area is enlarged, the etching recipes are not easily controlled.